1. Field of the Invention
The present invention relates to a cell search method in a mobile communication system and a mobile station, and more specifically, to a cell search method for a mobile station in a CDMA (Code Division Multiple Access) mobile communication system and a mobile station, the method comprising a three-step cell search method of continuing to execute a first step during a second or third step concurrently therewith.
2. Description of the Related Art
In a mobile communication system based on the CDMA method, if a mobile station communicates with a base station or measures power received from a base station, it must detect frame boundaries and a scramble code in a down signal from the base station. This is called “cell search”.
The cell search method essentially comprises descrambling the signal at all possible scramble codes with all possible timings. Then, a timing and a scramble code is selected with which a correlation coefficient obtained as a result of despreading with spreading codes used is largest, thereby making it possible to detect frame boundaries and the scramble code for the base station. This method, however, requires a large amount of time for the cell search. Thus, to increase the speed of the cell search, a method is used in which the base station transmits each slot through two synchronization channels: a PSCH (Primary Synchronization CHannel) and an SSCH (Secondary Synchronization CHannel) (refer to 3GPP Technical Specification 25.211).
FIG. 1 shows a configuration of a down channel relating to the cell search. This down channel relates to the cell search in the W-CDMA method (refer to 3GPP Technical Specification 25.211), which is representative of the CDMA mobile communication method. On a primary synchronization channel, a spreading code PSC (Primary Synchronization Code) is used which is common to all cells and slots, and the signal is transmitted in accordance with slot cycles. On a secondary synchronization channel, different spreading codes SSC0 to SSCn−1 (Secondary Synchronization Codes) are used for the respective slots, and one frame constitutes a spreading code sequence and is repeatedly and cyclically transmitted. Different spreading code sequences are used for respective cells and are correlated with scramble code groups to which scramble codes used by the base station belong.
FIG. 2 shows a conventional three-step cell search method.
A mobile station first detects slot boundaries (step S201). On the primary synchronization channel, the spreading code PSC, which is common to all the cell and slots, is used. The mobile station inputs a received signal to a matched filter corresponding to this spreading code PSC, and executes averaging over a plurality of slots in order to reduce the adverse effects of noise and interference. Then, a timing with which an average correlation coefficient is largest is selected to detect slot boundaries. This operation will be hereinafter called a “first step”.
Next, frame boundaries and a scramble code group are detected (step S203). On the secondary synchronization channel, the different spreading codes are used for the respective slots, and a spreading code sequence of these codes constitutes one frame. The spreading code sequence is repeated in accordance with frame cycles, and different spreading code sequences are used for the respective cells. These spreading code sequences are correlated with respective groups of scramble codes so as to allow the scramble codes to be subsequently detected easily. Since the slot boundaries have been detected at the first step, the mobile station can calculate a transmission timing on the secondary synchronization channel.
The mobile station then despreads the received signal using the calculated timing and the spreading codes SSC. It then averages correlation output coefficients corresponding to all possible frame boundaries and SSC sequences, and selects a timing and an SSC spreading code sequence with which the average correlation coefficient is largest. The mobile station thus detects the frame boundaries and a scramble code group. This operation will be hereinafter called a “second step”.
The mobile station further detects a scramble code (step S205). The mobile station, which has detected the frame boundaries and the scramble code group during the second step, finally receives the signal, in which the respective cells are subjected to different scramble code, through a common pilot channel and determines which of the scramble codes of the scramble code group equals that of the signal. Since the frame boundaries have already been detected, the phase of the scramble code can be calculated.
Since the spreading code for the common pilot channel is common to all the cells, essentially all the scramble codes within the group is used to descramble the signal, and the spreading code for the common primary channel is used to despread the signal. Then, these operations are performed over a plurality of symbols with the results averaged, and a scramble code is selected with which the average correlation coefficient is largest. This operation will be hereinafter called a “third step”.
The mobile station determines whether or not the detected frame boundaries and scramble code are correct (step S207). If it is determined that they are correct, then the cell search is ended. Otherwise, the cell search is restarted from the first step. Whether or not the frame boundaries and scramble code are correct is determined by comparing these values with referential values.
In the conventional three-step cell search method, the process of the above described first, second, and third steps are serially executed to determine whether or not the detection results, that is, the frame boundaries and the scramble code are correct. If it is not determined that the detection results are correct, then memories for the respective steps are initialized, and the cell search is restarted from the first step. The above operation is repeated until the correct frame boundaries and scramble code are detected.
The power consumption of the mobile station can be reduced by shortening the time required for the mobile station to execute the cell search. Further, if the time required for the mobile station to execute the cell search is shortened, then the handover speed is increased to achieve more smooth and stable communications. Moreover, prompt and accurate selection of proper cells enables communications to be executed with a minimum required transmission power, thereby reducing the power consumption during the communication of the mobile station, while increasing the system capacity.
In the conventional three-step cell search method, however, the operations at the first to third steps are serially performed to determine that the cell search must be reexecuted, on the basis of only the determination for the detection results. Thus, the timing with which the need to reexecute the cell search is determined tends to be delayed.
In particular, with much noise or interference, there is a strong probability of a failure to detect the correct slot boundaries at the first step. If the detection fails at the first step, the operations at the second and third steps will be based on the incorrect slot boundaries and will thus be useless. Thus, the duration of the cell search disadvantageously increases.